Upon ingestion, most acid-labile pharmaceutical compounds must be protected from contact with acidic stomach secretions to maintain their pharmaceutical activity. To accomplish this, compositions with enteric-coatings have been designed to dissolve at a neutral pH to ensure that the drug is released in the proximal region of the small intestine (duodenum), rather than the acidic environment of the stomach. However, due to the pH-dependent attributes of these enteric-coated compositions and the uncertainty of gastric retention time, in-vivo performance as well as both inter- and intra-subject variability are all major set backs of using enteric-coated systems for the controlled release of a drug.
In addition, Phillips et al. has described non-enteric coated pharmaceutical compositions. These compositions, which allow for the immediate release of the pharmaceutically active ingredient into the stomach, involve the administration of one or more antacids with an acid labile pharmaceutical agent, such as a proton pump inhibitor. The antacid is thought to prevent substantial degradation of the acid labile pharmaceutical agent in the acidic environment of the stomach by raising the pH. See, e.g., U.S. Pat. Nos. 5,840,737 and 6,489,346.
A class of acid-labile pharmaceutical compounds that are administered as enteric-coated dosage forms are proton pump inhibiting agents. Exemplary proton pump inhibitors include, omeprazole (Prilosec®), lansoprazole (Prevacid®), esomeprazole (Nexium®), rabeprazole (Aciphex®), pantoprazole (Protonix®), pariprazole, tenatoprazole, and leminoprazole. The drugs of this class suppress gastrointestinal acid secretion by the specific inhibition of the H+/K+-ATPase enzyme system (proton pump) at the secretory surface of the gastrointestinal parietal cell. Most proton pump inhibitors are susceptible to acid degradation and, as such, are rapidly destroyed as pH falls to an acidic level. Therefore, if the enteric-coating of these formulated products is disrupted (e.g., trituration to compound a liquid, or chewing the capsule or tablet) or the antacid fails to sufficiently neutralize the gastrointestinal pH, the drug will be exposed to degradation by the gastrointestinal acid in the stomach.
Omeprazole is one example of a proton pump inhibitor which is a substituted bicyclic aryl-imidazole, 5-methoxy-2-[(4-methoxy-3,5-dimethyl-2-pyridinyl)methyl]sulfinyl]-1H-benzimidazole, that inhibits gastrointestinal acid secretion. U.S. Pat. No. 4,786,505 to Lovgren et al. teaches that a pharmaceutical oral solid dosage form of omeprazole must be protected from contact with acidic gastrointestinal juice by an enteric-coating to maintain its pharmaceutical activity and describes an enteric-coated omeprazole preparation containing one or more subcoats between the core material and the enteric-coating.
Proton pump inhibitors are typically prescribed for short-term treatment of active duodenal ulcers, gastrointestinal ulcers, gastro esophageal reflux disease (GERD), severe erosive esophagitis, poorly responsive symptomatic GERD, and pathological hypersecretory conditions such as Zollinger Ellison syndrome. These above-listed conditions commonly arise in healthy or critically ill patients of all ages, and may be accompanied by significant upper gastrointestinal bleeding.
It is believed that omeprazole, lansoprazole and other proton pump inhibiting agents reduce gastrointestinal acid production by inhibiting H+/K+-ATPase of the parietal cell the final common pathway for gastrointestinal acid secretion. See, e.g., Fellenius et al., Substituted Benzimidazoles Inhibit Gastrointestinal Acid Secretion by Blocking H+/K+-ATPase, Nature, 290: 159-161 (1981); Wallmark et al., The Relationship Between Gastrointestinal Acid Secretion and Gastrointestinal H+/K+-ATPase Activity, J. Biol. Chem., 260: 13681-13684 (1985); and Fryklund et al., Function and Structure of Parietal Cells After H+/K+-ATPase Blockade, Am. J. Physiol., 254 (1988).
Proton pump inhibitors have the ability to act as weak bases that reach parietal cells from the blood and diffuse into the secretory canaliculi. There, the drugs become protonated and thereby trapped. The protonated compound can then rearrange to form a sulfenamide, which can covalently interact with sulfhydryl groups at critical sites in the extra cellular (luminal) domain of the membrane-spanning H+/K+-ATPase. See, e.g., Hardman et al., Goodman & Gilman's The Pharmacological Basis of Therapeutics, 907 (9th ed. 1996). As such, proton pump inhibitors are prodrugs that must be activated to be effective. The specificity of the effects of proton pump inhibiting agents is also dependent upon: (a) the selective distribution of H+/K+-ATPase; (b) the requirement for acidic conditions to catalyze generation of the reactive inhibitor; and (c) the trapping of the protonated drug and the cationic sulfenamide within the acidic canaliculi and adjacent to the target enzyme. See, e.g., Hardman et al.